The present invention relates to an improvement in a disc brake pad which is incorporated in a disc brake which is used to apply the brakes of a vehicle such as a motor vehicle and a disc brake assembly. Specifically, the invention relates to a realization of a construction which can suppress effectively the generation of abnormal noise called brake squeal even in a case where a pressure exerted on a brake pad by a piston is weak as when the vehicle is slowed while driven at low speeds.
A disc brake assembly is widely used to apply the brakes of a motor vehicle. When the brakes are applied by the disc brake assembly, a pair of pads which are provided so as to hold a rotor which rotates together with a wheel are pressed against both side surfaces of the rotor. An opposed-piston type disc brake assembly shown in FIG. 15 or a floating caliper-type disc brake assembly shown in FIG. 16 is widely used traditionally as the disc brake assembly described above.
Of these brake assemblies, in the opposed piston-type disc brake assembly shown in FIG. 15, a caliper 4 having an outer body 2 and an inner body 3 is provided in a position where the outer body 2 and the inner body 3 hold a rotor 1 therebetween, and an outer cylinder and an inner cylinder are provided in the outer and inner cylinders, respectively, so that respective opening portions face opposite to each other. Additionally, an outer piston and an inner piston are fitted in the outer cylinder and the inner cylinder, respectively, in an oil-tight fashion so as to be displaced in an axial direction. An outer pad and an inner pad are held in the outer body 2 and the inner body 3, respectively, so as to be displaced individually in the axial direction. When the brakes are applied, hydraulic oil is sent into the outer cylinder and the inner cylinder under pressure, so that the outer pad and the inner pad are pressed against inner and outer side surfaces of the rotor 1 by the outer piston and the inner piston.
When referred to in this specification and claims, axial direction, circumferential direction and radial direction denote axial direction, circumferential direction and radial in relation to a rotor in such a state that a disc brake pad is incorporated in the disc brake assembly, respectively, unless otherwise described. Further, a rotor entrance side denotes a side where the rotor which rotates together with a wheel enters the caliper 4, and a rotor exit side of the caliper 4 denotes a side where the rotor exits from the caliper 4.
On the other hand, in the disc brake assembly shown in FIG. 16 which has the floating caliper, a caliper 4a is supported at supports 5 which are provided so as to lie adjacent to one side of a rotor 1 in such a way as to be displaced in an axial direction. Additionally, a pair of pads 6, 6 which are disposed on axial sides of the rotor 1 are also supported at the supports 5 so as to be displaced in the axial direction. A cylinder portion 7 and caliper claws 8 are provided on the caliper 4a so as to hold both the pads 6, 6 therebetween from both axial sides. Of these caliper constituent portions, a piston 9 is incorporated in the cylinder portion 7 so as to press the inner pad 6 (which is situated nearer to a middle of the vehicle in a widthwise direction in such a state that the caliper 4a is assembled to the vehicle, that is, the lower pad in FIG. 16) against the rotor 1. When applying the brakes, oil is sent into the cylinder portion 7 under pressure, so that the inner pad 6 is pressed against an inner side surface of the rotor 1 from bottom to top in FIG. 16 by the piston 9. Then, the caliper 4a is displaced downwards in FIG. 16 as a reaction to the pressing force exerted on the pad 6 by the piston 9, whereby the caliper claws 8 presses the outer pad 6 (which is situated outers of the vehicle in the widthwise direction in such a state that the caliper 4a is assembled to the vehicle, that is, the upper pad in FIG. 16) against the outer side surface of the rotor 1. As a result of this, the rotor 1 is strongly held on the inner and outer side surfaces thereof by the pads 6, this applying the brake.
Even with the opposed piston-type disc brake assembly shown in FIG. 15 and the floating caliper-type disc brake assembly shown in FIG. 16, it is known that the postures of the pads are made unstable when the brakes are applied, thereby causing abnormal noise called brake squeal. Then, to suppress the generation of such abnormal noise, various constructions have been proposed traditionally as described in Patent Documents 1 to 5, for example. FIGS. 17 and 18 show an example of a disc brake assembly 1 which incorporates therein disc brake pads of a conventional construction which is almost the same as that of the disc brake assembly described in Patent Document 1.
In the case of the depicted construction, a pad 6a includes a lining 10 and a metallic shoe or back plate 11 which is attached to be supported on a rear surface of the lining 10. A pair of projecting lug portions 12a, 12b are provided individually at radially middle portions of circumferential side edge portions of the back plate 11 so as to project in a circumferential direction from the circumferential side edge portions. Additionally, flat torque transfer surfaces 13a, 13b are provided individually on the circumferential side edge portions at portions which are situated further radially inwards than the lug portions 12a, 12b. Further, the lug portions 12a, 12b and the torque transfer surfaces 13a, 13b are continuously connected to each other by recess portions 14a, 14b where the back plate 11 is recessed towards a middle side of the pad 6a. 
On the other hand, a pair of guide portions 16a, 16b are provided individually on a pad supporting member 15 which supports the pads 6a so as to move in an axial direction. Then, guiding recessed grooves 17a, 17b are formed in radial middle portions on side surfaces of both the guide portions 16a, 16b which face each other in a circumferential direction, and flat torque bearing surfaces 18a, 18b are formed at portions which are situated further radially inwards than the guiding recessed grooves 17a, 17b. The pad supporting member 15 corresponds to a caliper of an opposed piston-type disc brake assembly or to a support of a floating caliper-type disc brake assembly.
Then, the lug portions 12a, 12b are brought into loose engagement with the guiding recessed grooves 17a, 17b in interiors thereof, respectively, and the torque transfer surfaces 13a, 13b are disposed opposite to the torque bearing surfaces 18a, 18b, respectively, whereby the pads 6a are supported on the pad supporting member 15 so as to move in the axial direction.
When a rotational direction of the rotor while the vehicle is traveling forwards is counterclockwise as seen in FIG. 17, a brake tangential force F {=μ (pad friction coefficient)×S (cylinder area)×P (cylinder hydraulic pressure)} is exerted on a center point A of a frictional surface of the lining 10. This moves the pad 6a towards a rotor exit side (a trailing side, a left hand side in FIG. 17), whereby the torque transfer surface 13a and the torque bearing surface 18a at the rotor exit side are brought into abutment with each other. In the case of the conventional construction, the torque transfer surface 13a and the torque bearing surface 18a are configured as flat surfaces on imaginary planes which are at right angles to an application line of the brake tangential force F, and therefore, the torque transfer surface 13a and the torque bearing surface 18a are brought into abutment (surface abutment) with each other over the whole surfaces thereof. Because of this, a reaction force is exerted on the brake pad 6a at a point B which is a radially central position of the torque transfer surface 13a which is spaced away (offset) radially inwards by a distance L0 from the point A which is the point of action of the brake tangential force F. Consequently, a moment M0 {=F (brake tangential force)×L0 (distance between A and B)} is exerted on the pad 6a, whereby the pad 6a is rotated counterclockwise. This applies a radially inward pressing force (a couple of force) Q0 to the lug portion 12a of the lug portions 12a, 12b which is situated at the rotor exit side, and this pressing force attempts to press a radially inner surface of the lug portion 12a against a radially inner surface of the guiding recessed groove 17a. Then, the pad 6a is rotated further counterclockwise while the radially inner surface of the rotor exit side lug portion 12a is dragged to the rotor exit side in relation to the radially inner surface of the guiding recessed groove 17a in such a state that the pressing force Q0 is exerted on the pad 6a. Then, finally, a radially outer surface of the lug portion 12b which is situated at the rotor entrance side is brought into abutment with a radially outer surface of the guiding recessed groove 17b with which the lug portion 12b is in engagement.
As has been described above, in the case of the conventional construction, the back plate 11 which makes up the pad 6a can be supported (restrained) on the pad supporting member 15 at a total of three locations including the torque transfer surface 13a at the rotor exit side, the radially inner surface of the lug portion 12a at the rotor exit side and the radially outer surface of the lug portion 12b at the rotor entrance side. This enables the posture of the pad 6a to be stabilized when the brakes are applied, thereby making it possible to suppress the generation of abnormal noise called brake squeal.
However, in the case of the conventional construction that has been described above, it becomes difficult to suppress the brake squeal in the event that the pressing force exerted on the pad 6 by the piston is weak when the brakes are applied while the vehicle is being driven at low speeds.
Namely, in the case of the conventional construction, since the radially inner surface of the lug portion 12a at the rotor exit side is formed into the flat surface, the contact point between the radially inner surface of the lug portion 12a and the radially inner surface of the guiding recessed groove 17a is made unstable, and a corner portion 19 which resides at a distal edge portion of the radially inner surface of the lug portion 12a tends to strike easily the radially inner surface of the guiding recessed groove 17a at its edge (tends to be easily caught). Because of this, as shown in FIG. 18, a frictional resisting force W0 {=μn (pad friction coefficient)×Q0 (pressing force)} acting in the direction of the rotor entrance side is exerted on a point C which is a longitudinal central position (a circumferential central position) of the radially inner surface of the lug portion 12a when the pad 6a slides towards the rotor exit side. At the same time, a push-up force f acting radially outwards is exerted on the corner portion 19. Of these forces, the frictional resisting force W0 generates a moment rA0 {=W0 (frictional resisting force)×X0 (distance between B and C)} which attempts to rotate the pad 6a in an opposite direction (clockwise) to the moment M0 which is based on the brake tangential force F about a center line which passes through the point B which is the radially central position of the torque transfer surface 13a at the rotor exit side. On the other hand, the push-up force f generates a moment rB0 {=f (push-up force)×K (distance from point C to corner portion 19)} which attempts to rotate the pad 6a in an opposite direction to the moment M0 about a center line which passes through the point C which is the longitudinally central position of the radially inner surface of the lug portion 12a. Because of this, a moment R0 (rA0+rB0) becomes larger which acts to cancel the moment M0 which attempts to rotate the pad 6a counterclockwise by such an extent that the corner portion 19 becomes easy to be caught by the radially inner surface of the guiding recessed groove 17a (by the magnitude of rB0). Consequently, it becomes difficult to rotate the pad 6a counterclockwise (M0−R0 becomes smaller), whereby, it becomes difficult to bring the radially outer surface of the lug portion 12b at the rotor entrance side into abutment with the radially outer surface of the guiding recessed groove 17b. In particular, when the pressing force exerted on the pad 6a by the piston is weak, since the moment M0 it self becomes smaller as the brake tangential force F decreases, it becomes difficult to bring the radially outer surface of the lug portion 12b at the rotor entrance side 12b into abutment with the radially outer surface of the guiding recessed groove 17b. As a result, it becomes difficult to support the pad 6a on the pad supporting member 15 at the three points (generating a state in which the pad 6a is supported at two points), whereby the posture of the pad is made unstable, which facilitates the generation of brake squeal.
In addition, in the case of the conventional construction, since the torque transfer surface 13a at the rotor exit side is formed into the flat surface, the point B which is the radially central position of the torque transfer surface 13a constitutes the center of the moment rA0, and the distance X0 to the application line of the frictional resisting force W0 becomes large. This increases the moment the moment rA0, whereby it becomes difficult to rotate the pad 6a counterclockwise based on the moment M0 which is based on the brake tangential force F. As a result of this, when the pressing force exerted on the pad 6a by the piston is weak, the brake squeal becomes easy to be generated. Additionally, since the torque transfer surface 13a at the rotor exit side is formed into the flat surface, in particular, when the pressing force exerted on the pad 6a by the piston is weak, the contact state between the torque transfer surface 13a and the torque bearing surface 18a is made unstable easily (they become loose to rattle). Consequently, the posture of the pad 6a becomes unstable due to this reason, whereby brake squeal is easy to be generated.
[Patent Document 1] JP-A-8-135696
[Patent Document 2] JP-A-2000-27905
[Patent Document 3] JP-A-2004-278646
[Patent Document 4] JP-A-11-63035
[Patent Document 5] JP-A-2001-304310